1. Field of the Invention
The present invention relates to a color image forming apparatus, and more particularly to a color image forming apparatus realized in a compact desktop size by reducing a total height while securing a sufficient length necessary for a sheet path between an image transfer point and an image fixing point. Also, the present invention relates to a method of making the above-mentioned color image forming apparatus.
2. Discussion of the Background
In recent years, an electrophotographic image forming apparatus has been increasingly demanded in a full-color version, such as a color printer, a color copying machine, and so forth. In response, quite a large number of full-color image forming apparatuses have been introduced to the market. In comparison with a monochrome image forming apparatus, a full-color image forming apparatus inevitably has larger dimensions, due to its structure, and achieves a relatively lower performance in image forming, e.g., a lower image forming speed. However, there is also a great demand for the full-color image forming apparatus to have a compact size, such as the monochrome printer, capable of being placed on a desk and to be able to perform at a relatively high image forming speed.
In the full-color image forming apparatus, there are two adoptable color recording methods; a single drum type and a tandem drum type. The single-drum-type image forming apparatus has a typical configuration in which a plurality of development units are arranged around a single photosensitive drum. The development units contain different color toners and sequentially transfer the color toners to the surface of the photosensitive drum so as to form a composite color image. The composite color image is then transferred onto a recording sheet. On the other hand, the tandem-drum-type image forming apparatus has a plurality of photosensitive drums arranged in line and forms single-color toner images with different color toners on the corresponding photosensitive drums. Then, the single-color toner images are sequentially transferred onto a recording sheet so as to form a composite color toner image.
The single-drum type has advantages in size and cost, in comparison with the tandem-drum type, but also has difficulty in enhancing the image forming speed due to the need to repeat image forming, which is normally repeated four times. On the contrary, the tandem-type has disadvantages in size and cost, but has an advantage in the enhancement of the image forming speed.
Under the aforementioned circumstances, increasing attention has been focused on full-color image forming apparatus based on the tandem drum type, to attain high speed image forming like the monochrome printer.
There are two different types of tandem-drum image forming apparatuses, as shown in FIGS. 1 and 2. In the tandem-drum image forming apparatus shown in FIG. 1, images formed on four photosensitive drums 51, arranged in line, are sequentially transferred by corresponding image transfer units 52 onto a recording sheet, which is conveyed from a sheet supply unit 60 to an image fixing unit 61 by a sheet conveying belt 53. This method is referred to as a direct image transfer method. In the tandem-drum image forming apparatus shown in FIG. 2, in which components equivalent to those shown in FIG. 1 are given the same numeral references, images formed on the four photosensitive drums 51, arranged in line, are sequentially transferred by corresponding primary image transfer units 52 to form a composite color image onto an intermediate transfer belt 54. Then, the composite color image carried by the intermediate transfer belt 54 is transferred by a secondary image transfer unit 55 onto a recording sheet, which is conveyed from a sheet supply unit 60 to an image fixing unit 61 by a sheet conveying belt 53. This method is referred to as an indirect image transfer method.
In the tandem-drum-type image forming apparatus of FIG. 1, which adopts the direct image transfer method, the sheet supply unit 60 and the image fixing unit 61 need to be arranged upstream and downstream, respectively, in a sheet conveying direction relative to the four-tandem-drum mechanism. Therefore, the apparatus using the direct image transfer method is inevitably upsized in the sheet conveying direction, which is a drawback of this type of apparatus. On the contrary, in the image forming apparatus of FIG. 2, which adopts the indirect image transfer method, the secondary image transfer unit 55 can be positioned rather freely and, thus, a transfer path for the recording sheet can be shortened. Therefore, it is possible to reduce the size of the apparatus by using the indirect image transfer method.
From the above explanation, a full-color image forming apparatus preferably has the tandem-drum-type from the viewpoint of high speed, and preferably adopts the indirect image transfer method from the viewpoint of downsizing.
In the full-color image forming apparatus using the tandem-drum mechanism and the indirect image transfer method, a vertically-extended sheet transfer mechanism can be employed to minimize a sheet travel distance, along the sheet transfer path, from a sheet inlet of the sheet supply unit to the fixing unit. In this instance, the speed of image forming can be enhanced by reducing the amount of the sheet travel distance. Further, with this structure, the occurrence of a deficiency such as a sheet jamming may be suppressed. In such an apparatus using the vertically-extended sheet transfer mechanism, the second image transfer unit 55 is necessarily positioned next to one end of the intermediate transfer belt 54 (e.g., next to the right of the intermediate transfer belt 54), as shown in FIG. 3.
In this instance, if four image forming mechanisms 50 including the photosensitive drums 51a are arranged in line on and along the upper running surface of the intermediate transfer belt 54, an overlaid composite color image is created on the intermediate transfer belt 54 when a black color toner (Bk) is transferred onto the intermediate transfer belt 54. The black color toner (Bk) is the last toner transferred in the image forming sequence and, therefore, the overlaid composite color image is brought close to the secondary image transfer unit 55 only after a half turn of the intermediate transfer belt 54. This makes the first copy time relatively long. The first copy time is one of the speed indicators for image forming apparatuses, and indicates a speed for copying a first page.
To improve the first copy time in the above-mentioned image forming apparatus, the four image forming mechanisms 50 are arranged on and along the lower running surface of the intermediate transfer belt 54, instead of on and along the upper running surface thereof, as shown in FIG. 4. FIG. 5 is a top view of the image forming apparatus of FIG. 4. With this structure, the length of the sheet transfer path is minimized and the first copy time is improved, since the overlaid composite color can be brought close to the secondary image transfer unit 54 immediately after the transfer of the black color toner (Bk) is completed.
As described above, based on the presently available techniques, a desk-top and high speed full-color image forming apparatus may be realized, most preferably by using the tandem-drum image forming mechanism, the indirect image transfer method, and the vertical sheet conveying path.
It should be noted that in an electrophotographic image forming apparatus, the sheet conveying path between the image transfer point and the fixing point needs to have a distance to a certain extent determined by the size of the sheets applied or the like. The reason for this is explained with reference to FIG. 6.
In FIG. 6, the secondary image transfer unit 55 has a line speed b and the fixing unit 61 has a line speed a. Ideally, the line speeds a and b would be equal to each other. However, making the line speeds a and b equal to each other is not practical, in general, due to manufacturing tolerances, even if they are designed to be equal to each other. When the line speed b of the image transfer is slower than the line speed a of the image fixing, the leading edge of the recording sheet may reach the fixing unit 61 before the rear part of the recording sheet passes by the image transfer unit 55, depending upon the size of the recording sheet.
In this case, the recording sheet under the image transfer process is forcibly pulled forward by the fixing unit 61 and, as a result, image displacement is caused. To avoid this, the line speed b is generally designed to be faster than the line speed a. However, when the line speed b is faster than the line speed a, the recording sheet may have slack or a bend that causes the toner image on the recording sheet to contact a part of the machine. As a result, the toner image on the recording sheet is disturbed.
Therefore, the sheet passage between the image transfer unit 55 and the fixing unit 61 should have a length h that can accommodate slack or a bend of the recording sheet. Based on this structure, a vertical distance (i.e., a height h sin β; see FIG. 7) from the image transfer point to the fixing point is determined to avoid the above-mentioned image displacement problem by satisfying relationships a≦b, (b−a)xc/b=1, and Bmax≦BBmax. In these relationships, a is the line speed of the fixing rollers, b is the line speed of the image transfer rollers, c is the length of the recording sheet in the sub-scanning direction, Bmax is a maximum amount of a slack or a bend of the recording sheet caused between the image transfer point to the fixing point, and Bbmax is a maximum permissible amount of a slack or a bend of the recording sheet caused between the image transfer point to the fixing point.
In a full color image forming apparatus employing tandem-drum-type image forming and indirect image transfer, as well as a vertical sheet conveying path, it is considerably difficult to decrease the total height of such apparatus while securing a reasonably sufficient distance between the image transfer point and the fixing point. If the full color image forming apparatus is a desk-top machine, it is generally required to have a smaller profile in every dimension. However, the most critical dimension is the height, since it directly affects the ability of the user to access the recording sheets in the ejection tray, to remove the jammed sheets, to exchange the toner cartridge, and so forth. The difficulty lies in the relationship between securing the certain distance between the image transfer point and the fixing point, and in reducing the machine height, which are contradictory objectives.